ABSTRACT Virtually all cellular processes, from DNA replication to cell signalling, depend on multisubunit protein complexes. Assembly of these complexes is highly regulated to ensure production of functional, stoichiometric macromolecules. Any orphan subunits, that are synthesized in excess or remain unassembled, must be recognized and degraded to maintain protein homeostasis. One class of proteins for which quaternary assembly poses a particular challenge is integral membrane proteins. This family includes hundreds of oligomeric ion channels, receptors, and transporters essential for all aspects of cell biology. Many membrane protein subunits contain regions that, while necessary for function or oligomerization, would be thermodynamically unfavorable in isolation within the lipid bilayer. However, very little is known about the molecular chaperones that therefore must stabilize unassembled subunits in the endoplasmic reticulum membrane, or how these factors triage clients towards a biosynthetic or degradative fate. I propose to identify factors responsible for membrane protein assembly and quality control using two complementary strategies: i) biochemical analysis of the interactome of a series of orphan subunits, and ii) a genome-wide knockout screen using a reporter for membrane protein assembly. The identified candidates will be further analyzed using a combination of functional and structural strategies to dissect the molecular mechanisms of membrane protein assembly. A systematic understanding of the factors that regulate this process will open a new area for research in protein biogenesis, with the potential for broad biological and biomedical impact.